The　wavelength　tenability　of　QPM-OPOs　has　been　theoretically　analyzed　based　on　the　Sellmeier　equation　of　periodically　poled　crystals　and　the　QPM　theory.　Furthermore,　tuning　curves　of　PPRTA,　PPKTP　and　PPLN　OPOs,　including　polar　inversion　grating　period　tuning,　temperature　tuning　and　pump　wavelength　tuning,　etc.　have　been　calculated　and　compared.　By　comparison　with　PPKTP　and　PPLN　crystals,　parametric　tuning　characteristics　of　PPRTA　crystal　have　been　superior　to　that　of　PPKTP　and　PPLN　crystals.　The　numerical　simulations　agree　with　the　experimental　data　reported　by　foreign　researchers.　Therefore,　PPRTA　crystal　has　been　approved　to　be　an　ideal　QPM　nonlinear　material　generating　tunable　midinfrared　coherent　radiation.